Use of a vinylidene fluoride copolymer for providing a film with properties of adhesion

ABSTRACT

The present invention relates to the use of a fluorinated copolymer in the manufacture of a solid polymer film, to give said film properties of adhesion to a metal surface or to glass. It also relates to a process for improving the adhesion of a fluoropolymer to a metal, polymer or glassy substrate, and also to a composite part comprising a solid polymer film in direct contact with at least one metal or glassy element.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.16/072,225, filed Jul. 24, 2018, which is the U.S. national stageapplication of International Patent Application No. PCT/FR2017/050101,filed Jan. 17, 2017.

FIELD OF THE INVENTION

The present invention relates to the use of a fluorinatedcopolymerfluorinated copolymer in the manufacture of a solid polymerfilm, to give said film properties of adhesion to a metal or polymersurface or to glass. It also relates to a process for improving theadhesion of a fluoropolymer to a metal, polymer or glassy substrate, andalso to a composite part comprising a solid polymer film in directcontact with at least one metal or glassy element.

TECHNICAL BACKGROUND

Metallized polymer films have many applications, in particular in themanufacture of electrically conductive devices. Among the polymers thatcan be used, fluoropolymers based in particular on vinylidene fluoride(VDF) represent a class of compounds that have remarkable properties fora large number of applications. PVDF and copolymers comprising VDF andtrifluoroethylene (TrFE) are particularly advantageous due to theirpiezoelectric properties. They can thus be used for the manufacture ofvarious pieces of electroactive equipment, such as actuators or sensors,in general comprising a film of the polymer sandwiched between twoelectrodes.

Conversely, it is known to apply a polymer film to a metal substrate, inparticular for the purpose of giving it corrosion resistance. It hasalso been suggested to use VDF-based polymers for this purpose, in sofar as they have good barrier properties and good resistance to climaticconditions.

It is understood that, in these various applications, one conditionessential to obtaining the desired result is the good adhesion of thepolymer to the metal.

This property is particularly important for the manufacture ofelectroactive devices. Specifically, the good adhesion of the polymer toa metal electrode makes it possible to simplify the process formanufacturing these devices by rendering the preliminary step oftreating the electrode (in particular using chromium), so as to promotethe adhesion of the electroactive polymer, unnecessary. It also enablesthe manufacture of multilayer devices without the risk of delaminationor loss of electrical conductivity.

However, it has been observed that VDF homopolymers and copolymers, inparticular copolymers of VDF and of trifluoroethylene (TrFE), exhibited,due to the hydrophobic nature thereof, insufficient adhesion to metals.

To solve this problem, it has been proposed to mix PVDF with copolymersthat improve its compatibility with metals, in particular methylmethacrylate copolymerized with monomers bearing phosphonic acidfunctions (C. Bressy-Brondino et al., J. Appl. Polym. Sci., 2002, 8,2277-2287). However, these additives modify the properties of the PVDFand in particular its dielectric activity. As a variant, it has beensuggested to insert a layer of these copolymers between the metalsubstrate and the PVDF film (US 2010/0057189). This approach is notsuitable either for the formation of electroactive devices, in which theelectroactive fluoropolymers must be in direct contact with the metalsurface in order to limit dielectric losses.

Another solution consisted in grafting acid monomers to PVDF previouslyoxidized by ozonation (Brondino et al., J. Appl. Polym. Sci., 1999, 72,611-620). This technique is however liable to result in a degradation ofthe polymer chains during the ozonation step. Similarly, it has beenproposed to copolymerize vinylidene fluoride with perfluorovinyl ethers(Yamabe et al., Euro. Polym. J., 2000, 36, 1035-1041) or with vinylesters such as vinyl acetate (WO 2014/149911) or else with an epoxymonomer of glycidyl ether type combined with a maleic acid monoesterused as crosslinking agent (EP 0 751 157). Although this approach makesit possible to improve the adhesion of the polymer, it has the drawbackof modifying its properties and in particular its electroactivityproperties.

Copolymers of VDF with monomers bearing phosphonic acid functions suchas vinylphosphonic acid (WO 2012/030784; US2012/0184653; WO 2014/162080)are furthermore known. It is not suggested that the copolymers describedin these documents have a remarkable adhesion to metals and/or to glass.Moreover, the vinylphosphonic comonomer represents at least 1 mol %, andpreferably 2 to 18 mol %, of the copolymer of WO 2014/162080. It is alsoknown that vinylphosphonic acid makes it possible to improve theadhesion of certain copolymers based on ethylene and ontetrafluoroethylene (U.S. Pat. No. 3,445,434) to a metal substrate.However, these copolymers have a very compact crystalline structure thatdoes not allow them to be shaped at ambient temperature. They musttherefore be applied in the molten state to the substrate.

Another adhesion-promoting monomer which has been copolymerized with VDFis trifluoromethacrylic acid (JP2003-040936). It is not howeversuggested that it can improve the adhesion of copolymers based on VDFand on TrFE to metal substrates or to glass, a fortiori when it isintroduced into these copolymers in a small amount, less than or equalto 1 mol %.

SUMMARY OF THE INVENTION

There remains the need to provide a simple, economically advantageousand efficient means for imparting a good adhesion of polymers based onVDF and on TrFE to polar hydrophilic surfaces such as metal surfaces andglass and to polymers, without significantly modifying the properties ofthese polymers and in particular their thermal properties and theirelectroactivity.

It has turned out that this need could be met by copolymerizing VDF andTrFE with a small amount of an adhesion-promoting monomer which consistsof a non-perfluorinated vinyl or vinylene monomer bearing at least oneweak acid or weak acid precursor function. It is thus possible toenvisage the use of these copolymers in the manufacture of polymer filmsintended to be joined to metal parts with a view to obtaining variouscomposite parts.

The invention thus relates to the use of a fluorinatedcopolymerfluorinated copolymer in the manufacture of a solid polymerfilm, in order to give said film properties of adhesion to a metalsurface or to glass, characterized in that said copolymer is obtainedby:

-   (a) radical copolymerization of monomers comprising, and preferably    consisting of: (i) vinylidene fluoride (VDF), (ii) trifluoroethylene    (TrFE), (iii) optionally at least one other fluoromonomer and (iv)    an adhesion-promoting monomer which is a non-perfluorinated vinyl or    vinylene monomer bearing at least one weak acid or weak acid    precursor function, with the exception of carboxyvinyl,    carboxyvinylene, 1-alkylcarboxyvinyl and 1-alkylcarboxyvinylene    monomers and precursors thereof, and-   (b) when they are present, conversion of the weak acid precursor    functions into weak acid functions.

Another subject of the invention is a process for improving theadhesion, to a metal, polymer or glassy substrate, of a fluoropolymerobtained from monomers comprising, or preferably consisting of,vinylidene fluoride (VDF), trifluoroethylene (TrFE) and optionally atleast one other fluoromonomer, characterized in that it consists inintroducing into said fluoropolymer units resulting from the radicalcopolymerization of a non-perfluorinated vinyl or vinylene monomerbearing at least one weak acid or weak acid precursor function, with theexception of carboxyvinyl, carboxyvinylene, 1-alkylcarboxyvinyl and1-alkylcarboxyvinyl ene monomers and precursors thereof, and from theconversion of the weak acid precursor function into a weak acid, when itis present.

Yet another subject of the invention is a composite part comprising asolid polymer film in direct contact with at least one metal, polymer orglassy element, characterized in that said film is manufactured from acopolymer obtained by:

-   (a) radical copolymerization of monomers comprising, and preferably    consisting of: (i) vinylidene fluoride (VDF), (ii) trifluoroethylene    (TrFE), (iii) optionally at least one other fluoromonomer and (iv)    an adhesion-promoting monomer which is a non-perfluorinated vinyl or    vinylene monomer bearing at least one weak acid or weak acid    precursor function, with the exception of carboxyvinyl,    carboxyvinylene, 1-alkylcarboxyvinyl and 1-alkylcarboxyvinylene    monomers and precursors thereof, so that the molar proportion of the    moieties derived from said adhesion-promoting monomer represents    less than 1% of the copolymer, and-   (b) when they are present, conversion of the weak acid precursor    functions into weak acid functions.

It was observed that the introduction of the aforementionedadhesion-promoting monomer into fluoropolymers based on VDF and on TrFE,in an amount as low as 1 mol % or even less, made it possible toconsiderably increase the adhesion of these polymers to metal surfaceswithout substantially modifying their thermal stability, especiallytheir 5% weight loss decomposition temperature, determined bythermogravimetric analysis, their dielectric properties and inparticular their Curie temperature, measured by differential scanningcalorimetry, and also their semicrystalline nature, determined by theirmelting temperature and their enthalpy of fusion. In addition, thesecopolymers have a polarization curve similar to that of polymers with noadhesion-promoting monomer, namely the same remanent polarization, thesame coercive field and the same hysteresis. The result of this is thatthe range of uses of these fluoropolymers, modified according to theinvention by the introduction of an adhesion-promoting monomer, is notlimited by the introduction of this monomer.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates the radical terpolymerization of VDF with TrFE andDMVP and the hydrolysis of the terpolymer obtained.

FIG. 2 is a ¹H NMR spectrum (recorded at 20° C. in acetone-d₆) thatmakes it possible to observe the various types of protons present in thepoly(VDF-ter-TrFE-ter-DMVP) terpolymer prepared according to Example 1(at the bottom) and poly(VDF-ter-TrFE-ter-VPA) terpolymer preparedaccording to Example 2 (at the top).

FIG. 3 is a ¹⁹F NMR spectrum (recorded at 20° C. in acetone-d₆) thatmakes it possible to observe the various types of fluorine atoms presentin the poly(VDF-ter-TrFE-ter-DMVP) terpolymer prepared according toExample 1.

FIG. 4 represents a TGA thermogram, at 10° C./min, in air, of thepoly(VDF-ter-TrFE-ter-DMVP) terpolymer prepared according to Example 1.

FIG. 5 represents the superimposed TGA thermograms, at 10° C./min, inair, of the poly(VDF-ter-TrFE-ter-DMVP) terpolymer prepared according toExample 1 and of a comparative copolymer with no DMVP moieties.

FIG. 6 represents a DSC thermogram of the poly(VDF-ter-TrFE-ter-DMVP)terpolymer prepared according to Example 1.

FIG. 7 represents the superimposed DSC thermograms of apoly(VDF-ter-TrFE-ter-VPA) terpolymer prepared according to Example 2(bottom curve), of a poly(VDF-ter-TrFE-ter-DMVP) terpolymer preparedaccording to Example 1 (middle curve) and of a comparative copolymerwith no DMVP moieties (top curve).

FIG. 8 is a ¹H NMR spectrum (recorded at 20° C. in acetone-d₆) thatmakes it possible to observe the various types of protons present in thepoly(VDF-ter-TrFE-ter-MAF) terpolymer prepared according to Example 3.

FIG. 9 is a ¹⁹F NMR spectrum (recorded at 20° C. in acetone-d₆) thatmakes it possible to observe the various types of fluorine atoms presentin the poly(VDF-ter-TrFE-ter-MAF) terpolymer prepared according toExample 3.

FIG. 10 illustrates the appearance of films prepared with apoly(VDF-co-TrFE) copolymer (on the left), a poly(VDF-ter-TrFE-ter-DMVP)terpolymer (in the centre) and a poly(VDF-ter-TrFE-ter-VPA) terpolymer(on the right), respectively, once applied to aluminium foil.

FIG. 11 is a ¹⁹F NMR spectrum (top) and ¹H NMR spectrum (bottom), inC₅D₅N, of the poly(VDF-ter-TrFE-ter-MAF) terpolymer from Example 4.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The invention will now be described in greater detail and non-limitinglyin the description which follows.

One subject of the invention is the use of a fluorinated copolymer inthe manufacture of a solid polymer film, to give said film properties ofadhesion to a metal or polymer surface or to glass.

The fluorinated copolymer used in this invention comprises a moietyderived from vinylidene fluoride (VDF) and a moiety derived fromtrifluoroethylene (TrFE). In addition, it optionally contains at leastone other moiety derived from a fluoromonomer, which may in particularbe chosen from: tetrafluoroethylene (TFE), chlorofluoroethylene (CFE),chlorotrifluorethylene (CTFE), hexafluoropropylene (HFP),trifluoropropene, tetrafluoropropene, chlorotrifluoropropene,hexafluoroisobutylene, perfluorobutylethylene, pentafluoropropene,perfluoroethers such as perfluoromethylvinyl ether (PMVE) andperfluoropropylvinyl ether (PPVE), and mixtures thereof. It is clearlyunderstood that all the geometric isomers of the aforementionedfluorinated compounds are included in the above terminologies, such as3,3,3-trifluoropropene, 2,3,3,3-tetrafluoropropene (or 1234yf),3-chloro-2,3,3-trifluoropropene (or 1233yf) and3-chloro-3,3,3-trifluoropropene. Preferably, when it is present, saidother fluoromonomer is chosen from CFE and CTFE. Moreover, it ispreferred that the copolymer according to the invention does not containa moiety derived from a non-fluorinated monomer, apart from optionallythe adhesion-promoting monomer described below.

This fluorinated homopolymer or copolymer on its own does not have goodadhesion properties to polar hydrophilic surfaces such as metal surfacesor glass. In order to give this polymer the desired adhesion properties,moieties derived from an adhesion-promoting monomer are introduced intothis polymer. In order to do this, the aforementioned monomers arecopolymerized, by radical copolymerization, with an adhesion-promotingmonomer which is a non-perfluorinated vinyl or vinylene monomer bearingat least one weak acid or weak acid precursor function. In the casewhere the adhesion-promoting monomer bears a weak acid precursorfunction, the copolymerization step is followed by a step of convertingthe weak acid precursor functions into weak acid functions.

The weak acid function of the adhesion-promoting monomer isadvantageously chosen from a carboxylic acid function and a phosphonicacid function. When this monomer bears several (generally two or three)weak acid functions, these may be identical or different. In onepreferred embodiment of the invention, the adhesion-promoting monomerbears a single weak acid function.

It should be noted that this monomer is different from a carboxyvinyl,carboxyvinylene, 1-alkylcarboxyvinyl or 1-alkylcarboxyvinylene monomerand precursors thereof. Thus, acrylic acid, methacrylic acid and estersthereof are not included among the adhesion-promoting monomers that canbe used in the present invention. In the case where theadhesion-promoting monomer bears a carboxylic acid function, it ispreferred for this carboxylic acid function to be borne on a vinylenemonomer bearing an electron-withdrawing group such as a trifluoromethylgroup.

According to one particular embodiment of the invention, theadhesion-promoting monomer bears at least one function that is aprecursor of a weak acid, in particular a precursor of carboxylic acidor, better still, a precursor of phosphonic acid. Such precursors are inparticular carboxylic acid salts and alkyl esters and phosphonic acidsalts and alkyl esters. In the present invention, it is preferred to usephosphonic acid alkyl esters. Examples of such functions are thephosphonic acid monoalkyl ester and dialkyl ester functions andpreferably phosphonic acid dialkyl ester functions, such as phosphonicacid methyl, ethyl or isopropyl esters, more particularly phosphonicacid methyl esters. A vinyl monomer bearing such functions is inparticular vinylphosphonic acid dimethyl ester (DMVP).

It is clearly understood that the adhesion-promoting monomer may bearboth a weak acid function and a weak acid precursor function, chosenfrom those described above.

The preferred adhesion-promoting monomers according to this inventionare chosen from a vinylphosphonic acid dialkyl ester, in particularvinylphosphonic acid dimethyl ester, vinylphosphonic acid and(2-trifluoromethyl)acrylic acid.

The molar mass of the adhesion-promoting monomer is for example between100 and 250 g/mol, preferably between 100 and 200 g/mol.

The adhesion-promoting monomer may be introduced into the fluoropolymerin any molar amount, provided that it does not substantially affect thethermal, mechanical or electrical properties of the copolymer. Forobvious economic reasons, the molar percentage of the moiety derivedfrom this monomer within the copolymer will not however generally exceedan amount sufficient to obtain the desired adhesion properties. Thus, itis preferred according to this invention that the molar proportion ofthe moieties derived from the adhesion-promoting monomer represents lessthan 1% of the copolymer, preferably from 0.2% to 0.9%, for example from0.5% to 0.8% of the copolymer.

In the case where the adhesion-promoting monomer is2-trifluoromethacrylic acid (MAF), the molar proportion of moietiesderived from this monomer may be calculated using the followingequation:

${\%\mspace{11mu}{MAF}} = {{{{nMAF}/\left( {{nVDF} + {nTrFE} + {nX}} \right)}\mspace{14mu}{{where}:n_{MAF}}} = {\frac{1}{3}{\int_{- 59}^{- 72}{CF}_{3}}}}$n_(TrFE) = ∫⁻¹⁹⁴⁻²²¹CFH$n_{VDF} = {{\frac{1}{2}{\int_{- 90}^{- 132}{CF}_{2}}} - n_{TrFE}}$

where ∫_(t) ^(j)CF_(x) denotes the integral of the signal attributed toCFx, ranging from i to j ppm, in ¹⁹F NMR spectroscopy. The molarproportion of moieties derived from other adhesion-promoting monomersmay be calculated in a similar manner. In the case of adhesion-promotingmonomers containing no fluorine atoms, the molar proportion of themoieties derived from this monomer is calculated by combining theresults of ¹H and ¹⁹F NMR analyses.

According to one embodiment, the copolymer according to the inventionhas the following composition (in moles):

-   -   a proportion of moieties derived from vinylidene fluoride of        from 40% to 95%, preferably from 50% to 85%;    -   a proportion of moieties derived from an additional        fluoromonomer of from 0% to 15%,    -   a proportion of moieties derived from trifluoroethylene of from        5% to 60%, preferably from 15% to 50%; and    -   a proportion of moieties derived from the adhesion-promoting        monomer of from 0.1% to 5%, preferably from 0.5% to 2% and more        preferentially from 0.5% to 0.8%,

the above molar proportions being relative to the sum of the moles ofthe moieties constituting the copolymer.

The copolymers used according to the invention are advantageously randomand linear.

The copolymerization reaction is generally carried out in the presenceof a radical initiator. This may for example be a t-alkyl peroxyestersuch as tert-butyl peroxypivalate (or TBPPI), tert-amylperoxypivalate, aperoxydicarbonate such as bis(4-tert-butyl cyclohexyl)peroxydicarbonate, sodium, ammonium or potassium persulfates, benzoylperoxide and derivatives thereof, a t-alkyl hydroperoxide such astert-butyl hydroxyperoxide, a t-alkyl peroxide such as tert-butylperoxide or a t-alkylperoxyalkane such as2,5-bis(tert-butylperoxy)-2,5-dimethylhexane. As a variant or inaddition, use may be made, as radical initiator, of an azo initiator ora redox system.

According to one embodiment, the copolymerization may be carried out inthe presence of a dispersant. This may for example be a water-solublecellulose derivative, such as alkyl celluloses or alkyl hydroxyalkylcelluloses, a paraffin, polyvinyl alcohols and mixtures thereof.

According to one embodiment, the copolymerization may be carried out inthe presence of a chain transfer agent that makes it possible to controlthe molar mass of the copolymer, in particular with a view tofacilitating the processing thereof. Molar mass control agents may forexample be alkyl acetates such as ethyl acetate, bisalkyl carbonatessuch as diethyl carbonate, ketones such as butan-2-one, thiols, alkylhalides, saturated alcohols such as isopropanol and alkanes such aspropane.

Finally, the reaction medium may comprise one or more pH adjusters.

According to a first embodiment, the copolymer used according to theinvention is prepared by a radical solution polymerization process,comprising a step of copolymerization of a reaction mixture offluoromonomers and adhesion-promoting monomer in the presence of aradical initiator in a solvent.

According to one particular embodiment:

-   -   the molar proportion of VDF in the reaction mixture is from 40%        to 95%, preferably from 50% to 85%;    -   the molar proportion of TrFE in the reaction mixture is from 5%        to 60%, preferably from 15% to 50%;    -   the molar proportion of additional fluoromonomer in the reaction        mixture is from 0% to 15%, and    -   the molar proportion of adhesion-promoting monomer in the        reaction mixture is from 0.1% to 5%, preferably from 0.5% to 2%        and more preferentially from 0.5% to 0.8%,        the molar proportions being relative to the sum of the moles of        the monomers.

According to one embodiment, the reaction mixture essentially consistsof, and preferably consists of, a mixture of vinylidene fluoride andtrifluoroethylene with the adhesion-promoting monomer and optionally atleast one other fluoromonomer, radical initiator, and solvent. Theexpression “essentially consists of” is understood to mean that itcontains at least 70 mol %, more preferentially at least 80 mol %, forexample at least 90 mol %, or even at least 95 mol %, of theseconstituents.

The reaction is carried out in a solvent, which is for example chosenfrom a halogenated organic solvent such as 1,1,1,3,3-pentafluorobutane,2,2,2-trifluoroethanol, hexafluoroisopropanol; acetonitrile; a ketonesuch as methyl ethyl ketone or cyclohexanone; a carbonate such asdimethyl carbonate; an ester such as methyl acetate, ethyl acetate;water and mixtures thereof.

According to one embodiment, the reaction mixture is heated to areaction starting temperature between 20° C. and 100° C. and preferablybetween 25° C. and 80° C. The initial pressure inside the autoclavevaries as a function of the solvent, the temperature of the reaction andthe amount of monomers. It is generally between 0 and 80 bar, forexample between 20 and 40 bar. The choice of the optimal temperaturedepends on the initiator that is used. Generally, the reaction iscarried out over a period equal to two to four times the half life ofthe initiator used, for example from 6 hours to 25 h, at a temperatureat which the half life of the initiator is between 1 and 10 hours.

The molar mass of the copolymer obtained by solution polymerization ispreferably from 5000 to 200 000 g/mol, more preferentially from 10 000to 150 000 g/mol.

According to another embodiment, said terpolymer is prepared by aradical suspension polymerization process, comprising a step ofcopolymerization of a reaction mixture of the monomers in the presenceof water, a radical initiator, optionally a dispersant and, optionally,a chain transfer agent.

The suspension process makes it possible to avoid the use of toxicsolvents and of fluorinated surfactants (of PFOA or PFOS type which arebioaccumulative, toxic and persistent) during the synthesis andpurification of the copolymer.

In the suspension process, the monomers are loaded into a stirredreactor containing deionized water, optionally a dispersant and,optionally, a chain transfer agent.

The reactor is then brought to the desired initiation temperature, thistemperature being maintained during the polymerization at a valuebetween 40° C. and 60° C. The initiator is then injected into thereactor in order to start the polymerization. The pressure is generallymaintained within the range from 80 to 110 bar by injecting deionizedwater or a mixture of monomers. The consumption of the monomers leads toa drop in pressure which is compensated for by a continuous supply ofwater. The pressure is thus maintained within a range extending from 80to 110 bar. The reactor is then cooled and degassed. The product isdischarged and recovered in the form of a suspension. This suspension isfiltered and the wet powder is washed and then dried.

The suspension polymerization process is simplified since it makes itpossible to continuously inject only water in order to maintain thepressure in the reactor.

According to yet another embodiment, the terpolymer used according tothe invention is prepared according to a radical emulsion polymerizationprocess.

In order to do this, an aqueous dispersion of the initiator stabilizedby the surfactant used to carry out the polymerization is advantageouslyprepared. It is preferred not to use a perfluorinated surfactant. Toproduce this dispersion, water, initiator and a small fraction of thetotal amount of surfactant are mixed in a disperser. It is thisdispersion that is added at the start of, then optionally during, thepolymerization. After loading the polymerization reactor with water,surfactant and optionally paraffin, the reactor is pressurized, afterhaving removed the oxygen, by adding thereto vinylidene fluoride aloneor as a mixture with the comonomers and is brought to the chosentemperature. Advantageously, the aqueous emulsion is polymerized at atemperature of from 50° C. to 130° C. Preferably, the polymerization iscarried out at an absolute pressure of from 40 to 120 bar. The reactionis started by adding the dispersion of initiator. During thepolymerization, VDF alone or as a mixture with the comonomers isoptionally added in order to maintain the pressure or in order to obtaina controlled pressure variation. The initiator is optionally added inincrements or continuously. A chain transfer agent (CTA) may optionallybe added at the start of or during the polymerization. In the lattercase, it may be introduced in increments or continuously. Afterintroducing the anticipated amount of mixture of monomers, the reactoris degassed and cooled and the latex is drained off.

The recovery of the polymer from the latex forms the finishingoperation. This essentially consists in coagulating the latex then indrying the coagulate in order to obtain a dry powder. The finishingoperation may also include a washing step. This washing step may, forexample, be carried out by introducing the latex, optionally diluted,into a coagulator where it is subjected to shearing in the presence ofair. Under the combined effect of these two actions, the latex isconverted into an aerated cream having a density lower than that ofwater. This cream is optionally washed countercurrently with deionizedwater, for example according to the process described in U.S. Pat. No.4,128,517 and EP 0 460 284. The drying may be carried out according toany industrial means known to a person skilled in the art. Inparticular, the coagulated latex or the cream can advantageously bedried in a spray dryer. Thus, at the outlet of the washing column orimmediately after the coagulation, the aerated cream is sent to astorage container before being pumped into a spray dryer which convertsit into a dry powder. This step of drying in a spray dryer may also beapplied to the initial, optionally diluted, latex, to the latexcoagulated for example by mechanical shearing with or without priordilution or else to the aerated cream.

Another emulsion polymerization process that can be used to prepare thecopolymer used according to the invention is the one described indocument U.S. Pat. No. 7,122,608.

At the end of the copolymerization reaction, the copolymers obtainedmust be hydrolysed in the case where the adhesion-promoting monomer usedcontains a weak acid precursor function, in order to convert this into aweak acid function. This hydrolysis reaction may be carried out usingconventional reactants and hydrolysis (dealkylation) conditions,especially using strong acids or bases, such as hydrochloric acid, usedhot (for example at 80-100° C.) or, in the case especially of phosphonicacid alkyl esters, by treatment with sodium bromide followed by anacidification step or more preferentially by reaction with a halogenatedsilane, such as bromotrimethylsilane, in an organic solvent such as THF,at a temperature of from 20° C. to 40° C., for example, followed by ahydrolysis step using methanol.

The copolymer used according to the invention has sufficient mechanicalproperties to enable it to be able to be formed into a film. This filmforming may be carried out for example: by extrusion; by casting of asolution of copolymer in an organic solvent; by spin coating of asolution of copolymer in an organic solvent; or by printing of asolution of copolymers in an organic solvent. The films thus obtained,after a drying step followed by a postcuring step, have good mechanicalproperties and can be stretched.

Before this film-forming step, it is possible to add various additivesto the copolymer, such as reinforcing fillers, conductive fillers suchas carbon nanotubes, conductive salts, piezoelectric particles such asBaTiO₃ nanoparticles, plasticizers, crosslinking agents, crosslinkinginitiators, triethoxysilanes and mixtures thereof. The copolymer mayalso be mixed with another polymer such as PVDF.

The copolymers used according to the invention further preferablysatisfy at least one criterion which qualifies them as electroactivepolymers, in particular they have a Curie temperature below 110° C.,preferably below 100° C., and a maximum dielectric constant of greaterthan 30.

Their melting temperature is generally between 110° C. and 160° C., moreparticularly between 115° C. and 155° C.

Due to their good adhesiveness to polar hydrophilic surfaces such asglass and more particularly metal surfaces, these copolymers may be usedas coatings for these surfaces or on the other hand as substrates formetal coatings. The expression “metal surfaces” is understood to meansurfaces consisting of or coated with metals, metal oxides or metalalloys. The metals considered may be chosen from steel, copper, gold,silver, nickel or aluminium, without this list being limiting. As avariant, the copolymers according to the invention may be used ascoatings for polymer substrates and especially mixtures of ionomers suchas the PEDOT:PSS mixture where PEDOT denotespoly(3,4-ethylenedioxythiophene) and PSS denotes poly(styrenesulfonate).

These films are useful for the manufacture of composite parts comprisinga solid polymer film of the copolymer in direct contact with at leastone metal, polymer or glassy element. This composite part may form anelectroactive device, such as an actuator, a sensor or an artificialmuscle; a membrane; a capacitor; a binder for lithium-ion batteries; ora component of a device for producing energy such as a fuel cell.

EXAMPLES

The following examples illustrate the invention without limiting it.

Measurement Techniques and Apparatus

Nuclear Magnetic Resonance (NMR). The NMR spectra are recorded on aBruker AC 400 machine, deuterated acetone is used as solvent. Thecoupling constants and the chemical shifts are respectively given inhertz (Hz) and in parts per million (ppm). The acquisition parametersfor the ¹H [or ¹⁹F] NMR are the following: angle of rotation 90° [30° ],acquisition time 4.5 s [0.7 s], pulse sequence 2 s, number of scans 8[128] and a pulse duration of 5 μs for ¹⁹F NMR.

Thermogravimetric Analyses (TGA). The TGA analyses are carried out on10-15 mg samples on a Q 50 TGA machine from TA Instruments in aluminiumpans. The temperature rise is performed at 10° C./min, in air between25° C. and 590° C.

Differential scanning calorimetry (DSC). The DSC measurements areobtained on 10-15 mg samples on a Netzsch DSC 200 F3 machine using thefollowing analysis cycle: cooling from ambient temperature to −50° C. at20° C./min, isotherm at −50° C. for 5 min, first rise from −50° C. to200° C. at 10° C./min, cooling from 200° C. to −50° C. at 10° C./min,isotherm at −50° C. for 3 min, second temperature rise from −50° C. to200° C. at 10° C./min and last cooling from 200° C. to ambienttemperature. The calibration was carried out with noble metals andverified with a sample of indium before the analysis. The Curietransition temperature and melting temperature are determined at themaximum of the endothermic peaks.

Example 1: Radical Terpolymerization of VDF with TrFE and DMVP

A poly(VDF-ter-TrFE-ter-DMVP) terpolymer was prepared according to thereaction scheme illustrated in FIG. 1 (1^(st) step).

In order to do this, a 100 ml Hastelloy autoclave is equipped with inletand outlet valves, a rupture disk, a manometer and a pressure sensorconnected to a computer to record the change in pressure as a functionof time. The autoclave is pressurized with 30 bar of nitrogen in orderto verify the absence of leaks. It then undergoes three vacuum-nitrogencycles that make it possible to eliminate any trace of oxygen. Afterinerting the reactor, 60 ml of a degassed solution containingdi(tert-butylcyclohexyl) peroxydicarbonate (617 mg, 1.6 mmol) anddimethylvinyl phosphonate (DMVP, 683 mg, 5.0 mmol) in dimethylcarbonate(DMC) were introduced into the reactor. The reactor is then cooled to−80° C. in order to introduce the gaseous monomers. Trifluoroethylene(TrFE, 17.0 g, 207 mmol) then vinylidene fluoride (VDF, 20.0 g, 313mmol) are transferred into the reactor and the amount of each monomer ismeasured by double weighing. After having loaded all the reactants, theautoclave is reheated to ambient temperature then heated to 48° C. Thereaction lasts 17 hours and a pressure drop of 22 bar is observedrelative to the 30 bar at the start of polymerization. After thereaction, the reactor is placed in an ice bath and degassed. The crude,viscous and colourless solution is transferred into a beaker and dilutedin 200 ml of acetone. This solution is precipitated from 4 litres ofcold pentane. The product obtained, a white solid, is dried at 80° C.under vacuum for 14 hours. The polymer obtained (34.5 g, yield=91%) ischaracterized by ¹H (FIG. 2), ¹⁹F (FIG. 3) and ³¹P NMR spectroscopy,SEC, TGA (FIG. 4) and DSC (FIG. 6).

As illustrated in FIG. 5, the curve obtained by TGA for this terpolymercan be superimposed on the one obtained for the corresponding copolymer,with no moieties derived from the DMVP monomer. It can therefore beconcluded therefrom that the introduction of this monomer does notmodify the thermal properties of the fluoropolymer which remain stableup to 300° C. with a decomposition temperature corresponding to 5%weight loss which is equal to 390° C.

Similarly, as shown in FIG. 7, the DSC curve of the terpolymer fromExample 1 is substantially identical to that of the comparativecopolymer, with a melting temperature of 150° C. and an enthalpy offusion of 21 J/g. The semicrystalline structure of the copolymer is nottherefore altered by the introduction of the adhesion-promoting monomer.Moreover, the terpolymer has the same electroactivity properties as thecomparative copolymer, with a Curie transition temperature of 66° C.,characteristic of passing from a ferroelectric phase to a paraelectricphase.

Example 2: Preparation of a Poly(VDF-Ter-TrFE-Ter-VPA) Terpolymer

The poly(VDF-ter-TrFE-ter-DMVP) terpolymer obtained according to Example1 was hydrolysed according to the reaction scheme illustrated in FIG. 1(2^(nd) step) in order to obtain a poly(VDF-ter-TrFE-ter-VPA) terpolymerin which the phosphonic ester functions are converted into phosphonicacid functions.

In order to do this, a 250 ml three-necked round-bottomed flask,equipped with a 50 ml dropping funnel, a water-cooled condenser and athermometer, is dried and flushed with nitrogen for 15 minutes. Itcontains 10.0 g of the terpolymer prepared in Example 1. A low nitrogenpressure in the assembly prevents any ingress of moisture. 60 ml of drytetrahydrofuran (THF) are introduced via the dropping funnel. Thereaction medium is placed in an ice bath and cooled to 4° C. 675 mg ofbromotrimethyl silane (TMSBr) are added dropwise over 15 minutes. Next,the reaction medium is gradually reheated to ambient temperature. After15 hours of reaction, 100 ml of methanol (MeOH) are introduced via thedropping funnel. The solution is vigourously stirred for 2 hours. Afterthe reaction, the solvents are evaporated off using a rotary evaporator.The solid thus obtained is dissolved in acetone and then precipitatedtwice from 2 litres of cold pentane. The white powder obtained (8.2 g,yield=82%) is dried under vacuum for 14 hours, then characterized by 41(FIG. 2), ¹⁹F and ³¹P NMR spectroscopy, TGA and DSC.

In FIG. 2, the comparison of the spectrum obtained for the terpolymerfrom Example 2 with that of the terpolymer from Example 1 shows adisappearance of the broad unresolved peak between 3.7 and 3.9 ppm,characteristic of the protons of the methyl groups of the DMVP andtherefore confirms the complete hydrolysis of the DMVP (vinylphosphonicacid dimethyl ester) units to VPA (vinylphosphonic acid).

Example 3: Radical Terpolymerization of VDF with TrFE and MAF

A 100 ml Hastelloy autoclave is equipped with inlet and outlet valves, arupture disk, a manometer and a pressure sensor connected to a computerto record the change in pressure as a function of time (FIG. 9). Theautoclave is pressurized with 30 bar of nitrogen in order to verify theabsence of leaks. It then undergoes three vacuum-nitrogen cycles thatmake it possible to eliminate any trace of oxygen. After inerting thereactor, 60 ml of a degassed solution containingdi(tert-butylcyclohexyl) peroxydicarbonate (180 mg) and2-trifluomethylacrylic acid (MAF, 0.7 g, 5 mmol) in dimethylcarbonate(DMC) were introduced into the reactor. The reactor is then cooled to−80° C. in order to introduce the gaseous monomers. Trifluoroethylene(TrFE, 14.0 g, 169 mmol) then vinylidene fluoride (VDF, 21.0 g, 328mmol) are transferred into the reactor and the amount of each monomer ismeasured by double weighing. After having loaded all the reactants, theautoclave is reheated to ambient temperature then heated to 48° C. Thereaction lasts 18 hours and a pressure drop of 22 bar is observedrelative to the 23 bar at the start of polymerization. After thereaction, the reactor is placed in an ice bath and degassed. The crude,viscous and colourless solution is transferred into a beaker and dilutedin 200 ml of acetone. This solution is precipitated from 4 litres ofcold water. The product obtained, a white solid, is dried at 80° C.under vacuum for 14 hours. The polymer obtained (30.1 g, yield=84%,VDF/TrFE/MAF molar composition=68/31/1) is characterized by ¹H (FIG. 8)and ¹⁹F (FIG. 9) NMR spectroscopy, TGA and DSC.

In FIG. 8, the signal of the protons of the MAF units is within thebroad unresolved peak characteristic of the protons of the VDF units,i.e. between 2.2 and 3.4 ppm. The broad unresolved peak between 5.1 and6.0 ppm is characteristic of the protons of the TrFE units. In FIG. 9,the signals between −63 and −71 ppm are characteristic of the CF₃ groupsof the MAF units. The signals between −90 and −135 ppm arecharacteristic of the CF₂ groups of the TrFE and VDF units. The signalsbetween −193 and −221 ppm are characteristic of the CFH groups of theTrFE units. The combination of FIGS. 8 and 9 makes it possible tocalculate the composition of the terpolymer obtained.

Example 4: Radical Terpolymerization of VDF with TrFE and MAF

The synthesis, in aqueous suspension, of a poly(VDF-ter-TrFE-ter-MAF)terpolymer was carried out in a 3 litre reactor. VDF (790 g, 12.3 mol)and TrFE (434 g, 5.29 mol) were transferred into the reactor preloadedwith 1500 g of water and a hydroxypropylmethyl cellulose stabilizer. Thereactor is heated to 48° C. then the radical initiator is introduced.The pressure of the reactor is maintained between 80 and 100 bar byinjecting deionized water. MAF (25 g, 0.179 mol) is diluted in 60 ml ofdeionized water. A third of this solution is injected: (i) at the startof the polymerization, (ii) after conversion of one third and (iii) oftwo thirds of the (VDF+TrFE) amount initially introduced. At the end ofthe reaction, the crude product is filtered and the fine white powderobtained is washed several times in deionized hot water (50° C.). Thefinal product is dried for 24 hours at 60° C. in a ventilated oven.

The polymer obtained is characterized by ¹H and ¹⁹F (FIG. 11) NMRspectroscopy, TGA and DSC.

In FIG. 11, the signal of the protons of the MAF units is within thebroad unresolved peak characteristic of the protons of the VDF units,i.e. between 2.2 and 3.4 ppm. The broad unresolved peak between 5.1 and6.0 ppm is characteristic of the protons of the TrFE units. The signalsobserved by ¹⁹F NMR between −63 and −71 ppm are characteristic of theCF₃ groups of the MAF units. The signals between −90 and −135 ppm arecharacteristic of the CF₂ groups of the TrFE and VDF units. The signalsbetween −193 and −221 ppm are characteristic of the CFH groups of theTrFE units. The combination of the spectra presented in FIG. 11 makes itpossible to calculate the composition of the terpolymer obtained.

Table 1 below summarizes the conditions of the radicalterpolymerizations of VDF with TrFE and DMVP or MAF and the propertiesof the polymers obtained according to Examples 1 to 4.

TABLE 1 Composition M_(n) (% mol) Yld (NMR) T_(d5%) T_(C) T_(m) ΔH_(f)Ex. VDF TrFE APM (%) (kg · mol⁻¹) (° C.) (° C.) (° C.) (J/g) Comp.Monomers 63 37 — 78 31 413 72 150 21 Polymer 65 35 — 1 Monomers 60 39 191 20 390 66 150 21 Polymer 60.3 38.7 1 2 Monomers — — — 82 20 364 65149 20 Polymer 63 36 1 3 Monomers 65 34 1 84 67 405 76 147 22 Polymer67.6 31.5 0.9 4 Monomers 69 30 1 76 115 428 99 150 23 Polymer 69.2 30.30.5

In this table, “monomers” indicates the percentage of each of themonomers relative to the initial mixture of monomers and “polymer”indicates the weight percentage of units resulting from these monomersin the polymer, measured by ¹⁹F NMR spectroscopy. In addition, “APM”denotes the adhesion-promoting monomer used (DMVP in Example 1, VPA inExample 2, MAF in Examples 3 and 4).

Yld denotes the yield of the reaction.

Mn represents the number-average molecular mass of the polymer.

T_(d5%) denotes the decomposition temperature of the polymer resultingin 5% weight loss.

T_(C) represents the Curie temperature as determined by differentialscanning calorimetry (DSC) at the maximum of the endotherm during thesecond temperature rise at 20° C./min.

T_(m) represents the melting temperature of the terpolymer, asdetermined by differential scanning calorimetry (DSC) at the maximum ofthe endotherm during the second temperature rise.

ΔH_(f) represents the enthalpy of fusion measured by differentialscanning calorimetry (DSC) during the second temperature rise.

As it emerges from this table, the semicrystalline and electroactivenature of the fluoropolymer, and also the thermal stability thereof, arenot substantially altered by the introduction of the MAF groups, likethat which was observed for the DMVP and VPA groups.

Example 5: Preparation of Films and Adhesion Tests

In order to characterize the improvement in the adhesion properties,thin films were prepared on aluminium substrates. In order to do this,1.00 g of the poly(VDF-ter-TrFE-ter-DMVP) terpolymer (Example 1) and1.00 g of the poly(VDF-ter-TrFE-ter-VPA) terpolymer (Example 2) aredissolved separately in 5.00 g of methyl ethyl ketone (MEK) at ambienttemperature. The viscous solutions are deposited on the substrates andthe solvent is evaporated at ambient temperature over 8 h. The filmsthus obtained are dried for 14 h at 80° C. under vacuum then postcuredat 110° C. for 4 h.

A comparative film is prepared under the same conditions, from apoly(VDF-co-TrFE) copolymer.

It is observed that the film prepared from the comparative copolymer(FIG. 10, left) detaches from the aluminium foil without cracking. Thefilm obtained using the terpolymer from Example 1 (FIG. 10, centre) doesnot adhere to the aluminium substrate; it detaches and cracks due to theshrinkage thereof during drying. On the contrary, the film prepared fromthe terpolymer from Example 2 (FIG. 10, right) adheres perfectly to thealuminium foil even when the latter is deliberately deformed (folded,rolled, etc.). The film obtained from this terpolymer therefore has goodadhesion properties.

A film was prepared in a similar manner from the copolymer from Example4. It adhesion properties were evaluated according to the standard ASTMD3359, by comparison with the control film based on thepoly(VDF-co-TrFE) copolymer. The copolymer according to the inventionhad good properties of adhesion to glass, classified as level 5 on thescale of the ASTM standard (which ranges from 0 to 5). Similar resultswere observed on metal substrates such as gold and aluminium.

We claim:
 1. A process of preparing a film-forming polymer with adhesionproperties to metal, polymer or glass surfaces, consisting ofsynthesizing a fluorinated copolymer by: (a) radical copolymerizingmonomers comprising: (i) vinylidene fluoride (VDF), (ii)trifluoroethylene (TrFE), (iii) optionally at least one otherfluoromonomer and (iv) an adhesion-promoting monomer, saidadhesion-promoting monomer being a non-perfluorinated vinyl or vinylenemonomer bearing at least one weak acid or weak acid precursor functionother than carboxyvinyl, carboxyvinylene, 1-alkylcarboxyvinyl, and1-alkylcarboxyvinyl ene monomers and precursors thereof, wherein themolar proportion of the moieties derived from said adhesion-promotingmonomer is less than 1% of the copolymer, and (b) conversion of the weakacid precursor functions into weak acid functions.
 2. The processaccording to claim 1, wherein the molar proportion of the moietiesderived from said adhesion-promoting monomer is from 0.2% to 0.9%. 3.The process according to claim 1, wherein the molar proportion of themoieties derived from said adhesion-promoting monomer is from 0.5% to0.8% of the copolymer.
 4. The process according to claim 1, wherein saidother fluoromonomer is selected from the group consisting of:tetrafluoroethylene (TFE), chlorofluoroethylene (CFE),chlorotrifluorethylene (CTFE), hexafluoropropylene (HFP),trifluoropropene, tetrafluoropropene, chlorotrifluoropropene,hexafluoroisobutylene, perfluorobutylethylene, pentafluoropropene,perfluoroethers.
 5. The process according to claim 1, wherein the weakacid function is selected from the group consisting of a carboxylic acidfunction and a phosphonic acid function.
 6. The process according toclaim 1, wherein the precursor of a weak acid function is selected fromthe group consisting of carboxylic acid salts, carboxylic acid alkylesters, phosphonic acid salts and phosphonic acid alkyl esters.
 7. Theprocess according to claim 6, wherein the precursor of the weak acidfunction is the phosphonic acid alkyl ester and conversion comprisesreaction with a halogenated silane, in an organic solvent followed byhydrolysis in methanol.
 8. The process according to claim 1, wherein theadhesion-promoting monomer is selected from the group consisting of avinylphosphonic acid dialkyl ester, vinylphosphonic acid and(2-trifluoromethyl)acrylic acid.
 9. A process for forming afluoropolymer for improved adhesion, to a metal, polymer or glassysubstrate, consisting of: radical copolymerizing vinylidene fluoride(VDF), trifluoroethylene (TrFE) and optionally at least one otherfluoromonomer, and a non-perfluorinated vinyl or vinylene monomerbearing at least one weak acid or weak acid precursor function otherthan carboxyvinyl, carboxyvinylene, 1-alkylcarboxyvinyl and1-alkylcarboxyvinylene monomers and precursors thereof; wherein themolar proportion of the moieties derived from said adhesion-promotingmonomer is less than 1% of the copolymer, and conversion of the weakacid precursor function into a weak acid, when it is present.